To be able to run ESXi in VMware Workstation or Player your hardware needs to meet certain requirements, e.g. the CPU has to support VT-x and you have to enable it in the BIOS. You also need sufficient memory for the ESXi VM (at least 2GB, better 2.5GB or more).
As mentioned before you should consider to upgrade to at least WS 7 or WS 8. WS 8 has built in support to run ESXi VMs.
interesting posts you may want to read:
André